Preparation And Properties Of UV-curable Hyperbranched Polyurethane Acrylates-Polyurethane Diacrylates/SiO₂ Hybrid Coatings

Due to the unique spherical-multibranched structure,hyperbranched polymer （HBP） has outstanding performance, such as low viscosity, high solubility and reactivity, good compatibility. Therefore, HBP can be applied to prepare high performance UV-curable hyperbranched polymer/inorganic hybrid coatings. However, there are few reports about HBP used as prepolymer in UV-cured hybrid coatings at present. In this work, UV-curable HBPUA-PUDA/SiO₂ hybrid coatings with high hardness, good flexibility and abrasive resistance were prepared by two parts: hyperbranched urethane acrylate-urethane diacrylates （HBPUA-PUDA） with different proportion of hard segment and soft segment was used as organic component, and vinyl-modified silica sol prepared by sol-gel method was used as inorganic component. The composition, structure, properties, photopolymerization kinetics and thermal degradation mechanism of HBPUA-PUDA/SiO₂ hybrid coatings were studied. The main research contents and achievements are listed as follow.Firstly, a series of UV-curable hyperbranched polyurethane acrylates （HBPUA） were synthesized from hyperbranched ester （B-H20）, isophorone diisocyanate （IPDI） and 2-hydroxyethyl acrylate （HEA）. The products were characterized by GPC, FTIR and 1H-NMR. The effect of molecular weight on the viscosity of prepolymer, UV-curing, mechanical properties of the cured films was studied. It was found that the optimal synthesis conditions of HBPUA was that the isocyanate adduct of IPDI/HEA was prepared with 0.5wt.% DBTDL as catalyst at 35℃for 150min, and then the adduct reacted with B-H20 at 70℃for 180min. The results showed that the viscosity of HBPUA was much lower than that of the linear PUA. The results of GPC showed that the molecular weight distribution index increased with the increase of molecular weight of HBPUA. The curing rate of HBPUA was fast and percentage conversion of C=C is 81%. With the increase of molecular weight of HBPUA, the pendulum hardness of the cured films increased, however, the flexibility and adhesion of the cured films decreased.Secondly, UV-curable hyperbranched polyurethane acrylates-polyurethane diacrylates （HBPUA-PUDA）was synthesized from the isocyanate adduct of IPDI/HEA and different proportion of B-H20 and PEG. The effect of molecular weight and content of PEG on the viscosity of prepolymer, UV curing and the mechanical properties of the cured films was studied. It was found that the reaction time of the second step and the viscosity of HBPUA-PUDA increased with the increase of content and molecular weight of PEG. The curing rate and conversion percentage of C=C of HBPUA-PUDA was lower than HBPUA’s. With the increase of content of PUDA, the flexibility, adhesion and impact strength of the cured films increased, however, the pendulum hardness of the films decreased, the comprehensive performance of HBPUA-PUDA30 was the best. The results of DSC analysis showed that there were two Tg in the cured films of HBPUA-PUDA, with the increase of the content of PUDA, the Tg,h of hard segment increased, but the Tg,s of soft segment decreased.Thirdly, A series of UV-curable HBPUA-PUDA/SiO₂ hybrid coatings were prepared from two parts: HBPUA-PUDA30 was used as organic components and the modified silica sol from TEOS and MPTMS was used as inorganic components. The effects of the ingredient on structure, interfacial compatibility, UV-curing and properties of hybrid films were studied. It was found that the gel time of HBPUA-PUDA/SiO₂ was 183d when the ratio of MPTMS toTEOS was 1/3. The curing rate of HBPUA-PUDA/SiO₂ was faster than HBPUA-PUDA and the conversion percentage of C=C was 5% higher than HBPUA-PUDA’s. HBPUA-PUDA/SiO₂ hybrid films had good interfacial compatibility and SiO₂ were uniformly distributed in organic matrix. Tg of HBPUA-PUDA/SiO₂ hybrid coatings increased by 18.4℃than HBPUA-PUDA, and the energe storage modulus also increased than HBPUA-PUDA. When the content of modified silicane sol was 30wt.%, HBPUA-PUDA/SiO₂ hybrid coatings had optimal properties with transmittance of 93.1%, pendulum hardness of 0.74, adhesion of 0 grade, flexibility of 4mm, impact strength of 43 kg.cm and abrasion resistance of only 14.8mg after 300 wear cycles.Finally, The thermal stability thermal degradation mechanism of the HBPUA-PUDA/SiO₂ films was studied by using thermogravimetric analysis coupled with Fourier transform infrared spectroscopy （TGA/FTIR）. The results showed that all films exhibited two degradation stages located at about 320 and 440°C corresponding to the degradation for hard segments of urethane-acrylate and the degradation for soft segment and polyester core, decomposition products in the first stage were CO₂, amine with lower molecular weight, isocyanate and so on, in the second stage there were aldehyde, ester with lower molecular weight and some delayed CO₂. The results from the analysis of TGA/FTIR also indicated that decomposition temperature increased with the increase of the content of SiO₂, decomposition temperature of the HBPUA-PUDA/SiO₂-30 film were 15°C higher than that of the pure polymer.